4.10 Defining orbital subspaces

In the SCF, MCSCF, and CI programs it may be necessary to specify how many
orbitals in each symmetry are occupied (or internal in CI), and which of
these are core or closed shell (doubly occupied in all CSFs). This
information is provided on the OCC, CORE, and CLOSED cards in the
following way:

OCC,
;
CORE,
;
CLOSED,
;
FROZEN,
;

where is the number of occupied orbitals (including core/frozen and closed), the number of core
orbitals, and is the number of closed-shell orbitals (including the
core orbitals) in the irreducible representation . In general, , and . It is assumed that these numbers refer to the
first orbitals in each irrep. FROZEN only exists in the MCSCF program and
denotes frozen core orbitals that are not optimized (note that in older MOLPRO versions
frozen core orbitals were denoted CORE).

Note that the OCC and CLOSED cards have slightly different
meanings in the SCF, MCSCF and CI or CCSD programs. In SCF and MCSCF, occupied orbitals
are those which occur in any of the CSFs. In electron correlation methods (CI, MPn, CCSD etc), however, OCC denotes the
orbitals which are occupied in any of the reference CSFs. In the MCSCF, FROZEN
orbitals are doubly occupied in all CSFs and frozen (not optimized), while
closed denotes all doubly occupied orbitals (frozen plus optimized). In the CI and CCSD
programs, core orbitals are those which are not correlated and closed
orbitals are those which are doubly occupied in all reference CSFs.

OCC, CORE and CLOSED directives are generally required in each program
module where they are relevant; however, the program remembers the most recently used
values, and so the directives may be omitted if the orbital spaces are not to be changed
from their previous values. Note that this information is also preserved across
restarts.
Note also, as with the WF information, sensible defaults are assumed for these
orbital spaces. For full details, see the appropriate program description.

The orbital spaces may also be defined outside command blocks, and then
the directive is treated as global, i.e., it is used in all subsequent programs.
Spaces specific to certain wavefunction types can be defined by specifiying the
program name with a CONTEXT option, e.g.,

OCC,4,2,1,CONTEXT=MULTI

Alternatively, the context can be appended to the directive name with an underscore. For
example

OCC_MULTI,4,2,1

is equivalent to the previous form.

Local input given within command blocks has preference over global input.